Report shows link between evaporation, ice and lake levels

Tuesday

Jan 21, 2014 at 9:39 AMJan 21, 2014 at 6:14 PM

By Jim.Hayden@hollandsentinel.com(616) 546-4274

The ice cover on Lake Michigan continues to grow as the temperatures continue to drop, spelling good news for water levels come spring and summer. A new report, though, shows the relationship between evaporation and changing water levels in the Great Lakes is more complex than once believed."In light of these new findings, continued long-term monitoring of Great Lakes evaporation and related hydrological processes is paramount for understanding and predicting the future impacts of climate variability and change on Great Lakes water levels,” said Don Scavia of the University of Michigan, one of the co-directors of the Great Lakes Integrated Sciences and Assessments Center a joint U.S.-Canadian research team. The group released the ice report on Tuesday.The study is the first coordinated effort to study evaporation across the Great Lakes, according to a University of Michigan news release.Lead report writer John Lenters of the Ann Arbor consulting firm LimnoTech said ice doesn’t simply act as a “cap” that limits evaporation in winter. It also cools lake temperatures and helps delay the onset of evaporation later, according to The Associated Press. Heavy evaporation can occur shortly before ice forms.LimnoTech oversees the buoy placed in Lake Michigan of Port Sheldon that records wind speed and direction, air temperature, relative humidity, pressure, wave height and period and surface water temperature. It also takes video and still images of the waves and has been popular online.The Port Sheldon buoy was not used to gather data for the new report, Lenters said, but buoys have “strong potential” to help estimate lake evaporation.The report also shows that although ice cover affects evaporation, the evaporation rates in the autumn help determine winter ice cover.Lenters studied data from Lake Superior and found that evaporation rates during December 2013, a cold month, were about 60 percent higher than they were in December 2011, a warmer month, according to the University of Michigan release.High Great Lakes ice cover requires a large amount of heat loss from the lakes in the preceding autumn and early winter to cool the water enough to form ice, the report said, so extensive ice cover is actually an indicator of high evaporation rates prior to a high-ice winter. Those high evaporation rates can negate any increase in water levels from ice cover.That might not be the case this season, the report said. The extensive ice cover expected as the cold temperatures continue could lead to cooler summer water temperatures and a later start to evaporation in the spring."Together with high spring runoff from this winter's heavy lake-effect snowfall, it would be reasonable to expect a healthy rise in Great Lakes water levels this year," Lenters said.Lake Michigan levels hit a record low in 2013, but have rebounded.“We’re focusing on using the data to improve computer models of lake evaporation. The impact of that, then, will be to improve forecasts of Great Lakes water levels, particularly in a changing climate,” he said.Meanwhile, another paper being released this week says the Great Lakes have ebbed and flowed on a fairly consistent 10-year cycle for much of the past century, although the steep decline that began in 1998 suggests the pattern might have been broken. That study, led by Carl Watras of the Wisconsin Department of Natural Resources, says the fluctuations have been influenced by atmospheric trends hatched as far away as the northern Pacific Ocean.Taken together, the studies confirm that climate is by far the biggest player in determining water levels, Watras said. Human actions make little difference, despite noisy debates over whether structures should be built to regulate flows between lakes or whether communities outside the watershed should be permitted to draw from them, he said.“The two big factors are precipitation and evaporation,” said Watras, who is also a research fellow with the University of Wisconsin’s Center for Limnology, whose paper is being published this week in the journal Geophysical Research Letters. “The question looking ahead is how they will behave as the climate evolves. Will we get more of both? Or will one begin to dominate?”— The Associated Press contributed to this report. Follow Jim Hayden on Twitter@SentinelJim.